It has been desired in many sectors that the output of the semiconductor laser be enhanced. A factor for hindering the output enhancement per single mode of the semiconductor laser is an exit surface fusion caused by the laser beam which is called catastrophic optical damage (COD). The COD is conspicuous especially in an AlGaAs laser. Paying attention mainly to a reduction of a power density of the laser by expanding a laser guided mode, a weakly guiding laser having a thin active layer or a separate confinement type laser known as a large optical cavity (LOC) structure has hitherto been examined.
Based on such a structure, however, a strong correlation exists between a refractive index and a bandgap of each mixed crystal in a variety of laser materials ranging from the AlGaAs system. It is therefore impossible to independently control a carrier confinement and an optical confinement in a waveguide.
In particular, the expansion of the guided mode requires the thin active layer in either the LOC structured laser or the weakly guiding laser for the output enhancement. Further, a wide active layer is needed for obtaining a high gain for oscillations in the expanded guided mode. A self-contradiction thus exists therein. As a matter of fact, a limit of the mode expansion in an epitaxial direction by the above-mentioned methods is approximately 1 .mu.m at the maximum. A limit of the output is on the order of 100 mW per single mode.
Besides, in the weakly guiding laser having the thin active layer, the guided mode in the laminated direction exhibits an exponential function profile. Hence, a radiation density in the active layer where the catastrophic optical damage is caused is high as compared with the whole beam intensity. This is disadvantageous for the output enhancement. Besides, the guided mode has tails drawn deeply in the clad layers, and hence there is needed a growth of the clad layers that is considerably thick for the expansion of the guided mode.
In addition, both the guided mode (near-field pattern) and a beam radiation angle (far-field pattern) deviate largely from the Gaussian beam conceived as ideal one. There exists a problem in terms of a convergence of the beams in multiple applications.
On the other hand, there have also been examined lasers based on a so-called window structure in which the vicinity of an exit surface where a COD may occur is made transparent to the laser emission beam and a structure where a carrier injection is not effected in the vicinity of the exit surface. Those structures generally present, however, such problems that the astigmatism increases in addition to a complicated manufacturing process.
Further, there has been made an attempt to manufacture the high-output laser in the single mode by an optical feedback between a multiplicity of semiconductor lasers. The problem is, however, that the device becomes complicated.
It is an object of the present invention in view of the fact that multi-layered thin films have been easily formed by the molecular beam epitaxial (MBE) method, the metal organic chemical vapor deposition (MOCVD) method, etc. in recent years to solve the problems inherent in the conventional weakly guiding lasers and the LOC structured lasers in terms of overcoming the dilemma in designing the device for controlling the guided mode, attaining the output enhancement, the low dispersion of the radiation beams and improving the beam profile.